Unless otherwise indicated herein, the materials described in this section are not prior art to the claims in this application and are not admitted to be prior art by inclusion in this section.
High surface area electrodes are commonly used in electrolytic supercapacitors and ultracapacitors. For example, activated charcoal is commonly used in electrochemical double layer supercapacitors (EDLCs). Also, a carbon nanotube “fur” has been described that has high surface area but is geometrically regular compared to porous carbon.
The present disclosure appreciates that there are several limitations with such strategies to high surface area capacitor electrodes. For example, in EDLCs using porous carbon, the stored charge conducts through the carbon surrounding a pore and through electrolyte at the pore, and charge stored near the pore is accessed through a short path with small electrolyte resistance. In contrast, charges deeper in the pore are accessed through a longer electrolyte path with significantly higher series resistance. This pore effect can increase charge/discharge time and decrease total capacitance.
In the above-mentioned “fur” example, the “fur” is typically coupled to one electrode and the adjacent nanotubes are at the same voltage. Therefore, the “fur” can also experience pore effects that limit the charge and discharge rates, and possibly also the amount of charge. In the “fur”, parallel nanotubes create intermediate voids which resemble pores (deep and surrounded by carbon of a single polarity). Such interconnected voids do not overcome the pore effect, because the interconnection is lateral while the pore effect relates to depth (the length of the nanotubes). In the “fur” electrode, there can be a trade-off between such undesirable pore effects, which tend to increase as the spacing of the nanotubes decreases and length increases, and the desirable total charge, which tend to increase as the spacing of the nanotubes decreases and length increases, due to increased electrode area.
As is further appreciated in light of the present disclosure, overcoming pore effects while implementing high surface area electrodes for supercapacitors and ultracapacitors can be a complex undertaking.